In recent years a great deal of effort has been expended in the area of polycarbonates in order to improve both physical and mechanical properties. Much, if not all, of this effort as disclosed by the below art has been directed to incorporating various additives as a means of improving such properties as thermostability, resistance to discoloration, greater impact resistance, melt viscosity stability, resistance to degradation of the polycarbonate, and various other properties. In fact, some of the art has been directed to controlling molecular weight as a means of providing at least uniform properties from batch to batch or in a continuous process for preparing polycarbonates. However, this control of molecular weight alone does not provide for improved properties.
Specifically, U.S. Pat. Nos. 2,970,131 and 3,173,891 disclose controlling the molecular weight of the polycarbonate from batch to batch. Both references clearly disclose that the pH of the reaction medium is maintained between 10.5 to about 11.5.
Control of molecular weight, which is well recognized in the polymer arts, is important and necessary in polymerization reactions in order to achieve from batch to batch the same polymer having essentially the same properties. However, this alone is not the total answer to achieve improved properties of resistance to thermal degradation and improved impact resistance. It has been determined that in addition to molecular weight control from batch to batch, there is a need to control the polydispersity index within each batch, since when controlling the polydispersity index or keeping it as low as possible, excellent properties within each batch is achieved. polydispersity index is the ratio of the weight average molecular weight to the number average molecular weight. By maintaining this ratio as low as possible and actually in the range of 2.0 to 3.0, improved properties are achieved.